A ROM array is a non-volatile type memory. Any data stored in bitcells of the ROM array are not lost on power down of the memory array and can be retrieved after power up. The bitcells of the ROM array may be programmed by mask programming. In a mask programmed ROM array, the data is physically encoded in the circuit and data is programmed during fabrication only. Thus, it is impossible to change the respective content of the bitcells after fabrication of the ROM array. This leads to a number of serious disadvantages.
Apart from the fact that it is only economical to buy a storage device comprising a mask programmed ROM array in large quantities, one major technical disadvantage is that the turnaround time between completing the design for a mask programmed ROM array and receiving the finished product is long. Furthermore, a storage device comprising a mask programmed ROM array is usually impractical for research and development work since designers frequently need to modify the contents of the bitcells of the ROM array as they refine a design. Another disadvantage is that if a storage device is shipped with a faulty ROM array, the only way to fix it is to recall the product and physically replace the ROM array in every unit shipped.
In spite of the above constraints, a storage device comprising a ROM array is still economical over other ROM technology such as for example designs based on PROM, EPROM or EEPROM technology, as cost per chip is still less.
In the future the semiconductor memory technology will change more and more to smaller structures, for example from 28 nm technology to 16 nm technology.